1. Field of the Invention
The present invention relates generally to detecting gases, and more particularly to methods of and apparatus for detecting trace amounts of target gases, such as natural gas, remotely along long-range paths in the free atmosphere, wherein the exemplary natural gas detection is enabled by simultaneously remotely detecting methane and ethane along the same long-range path in the free atmosphere.
2. Description of the Related Art
In the field of gas detection, attempts are made to detect one specific, or xe2x80x9ctargetxe2x80x9d, gas even though local conditions render such detection difficult. Such difficulty may be based, for example, on the fact that the target gas may only be present in xe2x80x9ctrace amountsxe2x80x9d, such as one or a few parts per billion (PPB). Moreover, the target gas may be mixed with, or present with, water vapor and/or undesired (non-target) background, or xe2x80x9ccompetitivexe2x80x9d, gases that may be in the same atmosphere as contains the target gas, for example. The background gases are referred to as xe2x80x9ccompetitivexe2x80x9d gases because there are overlaps in absorption spectra of the trace gases and the background gases, e.g., in the infrared absorption spectra of such gases.
Many industries require facilities to detect target gases, such that there is a general need for accurate, fast and cost-competitive detection of target gases. However, the natural gas pipeline distribution system is the largest chemical distribution system in the United States. As a result, although equipment for detecting target gases other than natural gas has wide application in the United States, for example, the natural gas pipeline industry has the greatest need for accurate, fast, and cost-competitive chemical leak detection equipment and methods. This need relates in part to regulations that require gas utilities to perform periodic surveys for natural gas leaks.
Initially, in target gas detection for the natural gas industry, there is a need to distinguish between natural gas as a target gas, and other combustible gases. The main constituents of natural gas are methane and ethane, with methane being the primary component. However, methane is produced by many natural biological sources, including animal and plant. Thus, if an elevated methane level is sensed, it does not necessarily mean that there is a natural gas leak. In contrast, there are no substantial natural ethane emission sources. However, ethane generally does not exceed twenty-percent of natural gas. As a result, ethane is both more difficult to detect, but is a better indicator of natural gas than methane. Thus, to have an optimal natural gas detector, there is a need for the detector to simultaneously detect both methane and ethane to assure that the detected gas is from a natural gas leak and not from a natural emission source.
This need to simultaneously and independently detect both ethane and methane is not met by current gas detection equipment. For example, flame ionization detectors (FID) cannot distinguish natural gas from such competitive gases. As a result, when currently available FID equipment is used in an attempt to detect natural gas, the FID equipment provides xe2x80x9cfalse natural gas alarmsxe2x80x9d based on the detection of leaking propane tanks, leaking gasoline cans, so-called xe2x80x9csewer gasxe2x80x9d, and all other combustible gases. A natural gas pipeline utility using the FID equipment must respond to each false natural gas alarm although there is in fact no natural gas leak. Another limitation of the FID equipment is that during the detection process, it is generally necessary to place the equipment very close to the ground and within a xe2x80x9ccloudxe2x80x9d of the target gas that is to be detected. As a result, the FID detection process is relatively slow, and FID equipment cannot be used at a place remote from the locale of the gas leak, for example.
Also, Fourier Transform Infrared (FT-IR) spectro-radiometers use an interferometer to determine the spectral content of light passing through the free atmosphere. However, the output of such FT-IR instruments is based on a combination of all of the gases that are optically xe2x80x9cactivexe2x80x9d (e.g., infrared absorptive). within the spectral region of the instrument. Thus, their temporal response is generally poor. Moreover, FT-IR systems are expensive, have very limited detection range through the free atmosphere, and cannot detect very low concentrations of target gases.
Further, in contrast to the FID and FT-IR techniques, tunable diode laser absorption Spectroscopy (TDLAS), laser absorption spectroscopy (LAS), and differential absorption laser-based radar (DIAL) all use laser emission sources that are narrow band. For example, the DIAL devices typically monitor only one or two very narrow spectral absorption lines. Laser-based techniques are more costly to manufacture, maintain and use compared to broadband techniques such as gas correlation radiometry (GCR). However, gas correlation radiometry (GCR) is generally a passive technique that relies on solar illumination or scattering, or on thermal emission background. Thus, GCR instruments do not have an active source of energy that is directed through the free atmosphere to the instrument. Further, while GCR instruments may be provided with filters that improve a signal to noise ratio by generally limiting the overall bandwidth of light admitted to a detector of the GCR instrument, such filters do not provide an optimized bandwidth around an optimized central bandpass wavelength. As a result, the sensitivity of such GCR instruments may be as low as 10 to 100 parts per million (PPM).
Moreover, the FT-IR, TDLAS, LAS, DIAL and GCR technologies provide separate background gas and target gas channels that are interrogated sequentially. That is, light transmitted along a path through the free atmosphere and then through the background channel may be detected by a detector first. After such detection, the light transmitted through the same path through the free atmosphere and then through the target gas channel is detected by the same detector. The resulting temporal, or sequential, spacing of the alternating detection of the background channel and the target gas channel may vary from 0.1 second to several minutes. That is, it generally takes more than 0.1 seconds for these systems to provide a complete data set consisting of a target gas absorption measurement and an atmospheric background measurement, and during that lime period, there may be changes in the atmospheric conditions along the path of the light. Thus, the light that is transmitted along the path and through the target gas channel may have been subjected to different atmospheric conditions along the light path (e.g., atmospheric turbulence and variability) than the light transmitted through the background channel. As a result, the accuracy of these instruments is subject to a sensitivity limitation when used in a dynamic atmosphere. Atmospheric turbulence and variability generally limit the ultimate sensitivity of these instruments in that the same value of instrument output provided at different times may not be based on the same amount of the target gas. Further, attempts to avoid such atmospheric-induced inaccuracies, e.g., attempts to distinguish between signals generated based on a target gas and on the varying atmospheric conditions, have generally been limited to situations in which light is transmitted only a few feet through a detection path that may contain the target gas to be detected. For example, it may be practical to provide known modulation imposed on light transmitted along a detection path that is only a few feet long from transmitter to detector. Given the few feet between the transmitter and the detector, a conductor may easily input the characteristics of the know modulation to the detector so that demodulation will be accurate. However, problems are faced in accurately demodulating the modulated light when the detection path must, for practical purposes, be hundreds or thousands of feet long.
In addition to the accuracy limitation due to limited sensitivity of these prior instruments that interrogate the separate background gas and trace gas channels sequentially, such instruments have limitations when attempts are made to use the instrument on a mobile platform, such as a truck or airborne vehicle. For example, by the very motion of the mobile platform, there is a dynamic, or different, atmosphere through which the light is transmitted. As a result, the problems of atmospheric turbulence and variability limit the ultimate sensitivity of the instrument used on this type of platform. Remote sensing systems mounted on moving platforms have the additional problem of variable surface reflectivity. Prior remote sensing instruments mounted on mobile platforms suffered additional sensitivity limitations due to the temporally sequential manner of monitoring separate background and trace gas channels.
Some have attempted to avoid these and other limitations of atmospheric turbulence and variability by isolating samples of the target gas in a closed chamber that is controlled to avoid turbulence and variability. However, the need to capture such samples from a possible location of a gas leak, and other time factors, render the local sampling of the target gas impractical for detecting trace amounts of target gases such as natural gas along miles and miles of pipeline, for example.
Another problem faced in detecting target gases is isolation of a signal representing the target gas from other signals caused by background emissions. For example, although some types of gas detectors modulate the light just before the light impinges on a light receiver to remove thermal emission due to a warm instrument housing, such modulation at the receiver does not remove other background emissions such as atmospheric turbulence, jitter, beam wander, changes in the index of refraction, or the constant thermal emission of the atmosphere and the Earth.
What is needed, then, in the general field of detecting gases, is a way to distinguish between the presence of one target gas and other gases that are normally present in the free atmosphere at the same time and in the same place as the target gas. What is also needed in target gas detection, such as for the natural gas industry, is to be able to distinguish between natural gas as a target gas and other combustible gases. Moreover, there is a need for an optimal natural gas detector that simultaneously detects both methane and ethane to assure that the detected gas is from a natural gas leak, so as to avoid false natural gas alarms based on the detection, for example, of the noted leaking propane tanks, etc. In addition, there is a need to increase detection distance, that is, to increase the distance from a target gas detection instrument to a remote location of the target gas that is to be detected, and to also increase the detecting speed of such instruments. As well, there is a need to provide an instrument that will have a high sensitivity independently of atmospheric turbulence and variability. Finally, there is a need to remove undesired influences from the target gas signal so as to isolate detector signals representing the target gas. Such undesired influences include, for example, stray atmospheric fluctuations, such as humidity, atmospheric turbulence, changes in the index of refraction, and beam path variation; stray back light, such as the constant thermal emission of the atmosphere and the Earth; and system influences, such as jitter, beam wander and drift, and variation of source illumination.
Broadly speaking, the present invention fills these needs by providing, for the general field of detecting gases, methods of and apparatus for distinguishing between a target gas and other gases that are normally in the free atmosphere at the same time and in the same place as the target gas. The present invention fills the needs for trace gas detection in the natural gas industry by an ability to distinguish between natural gas as a trace gas and other combustible gases. The present invention also provides a more optimal natural gas detector that simultaneously detects both methane and ethane to assure that the detected methane is from a natural gas leak, so as to avoid false natural gas alarms based on the detection, for example, of leaking propane tanks, etc. The present invention also fills these needs by substantially increasing the detection distance. That is, by the present invention the distance from the detection instrument to a location of the target gas that is to be detected may be up to about fifty-three hundred feet. As a result, mobile platforms, such as trucks and aircraft, may be used to carry the equipment of the present invention during high-speed remote monitoring along long detection paths. In addition, the equipment of the present invention is provided with high sensitivity independently of atmospheric turbulence and variability, and the above-described undesired influences are removed from the trace gas signal so as to isolate detector signals representing the trace gas.
In one embodiment of the present invention, apparatus is provided for detecting at least one target gas in the free atmosphere, and includes a source of light configured to direct light into a region in which the at least one target gas may be present. For detection, there is also at least one gas conelation radiometer for responding to light transmitted through the region. A modulator is included for modulating the light at the source, i.e., before the light is directed into the region. This source modulation is one aspect of removing the above-described undesired influences from the trace gas signal so as to isolate detector signals representing the trace gas.
The present invention also contemplates an embodiment in which the light directed into the free atmosphere is broadband light and the modulator is configured to output the broadband light having either modulated amplitude or modulated frequency.
In yet another embodiment of the present invention relating to such removal of the above-described undesired influences, at least two gas correlation radiometer channels are provided for response to a single beam of light after transmission of the beam through the region. A beam splitter separates the single beam of light into two beams and directs one of the two beams to a separate one of the channels of the gas correlation radiometer. In an optimal configuration, the two separate channels separately and simultaneously respond to the respective one of the two light beams for separately and simultaneously generating signals that together indicate that the target gas is in the free atmosphere through which the single light beam was transmitted.
Another embodiment of the present invention relates to achieving substantially the same instrument conditions for separate light detectors of the separate channels of the gas correlation radiometers. The light detectors have a response to light that is a function of the light received, detector ambient temperature, and a bias voltage input to the detector. A voltage supply is configured to provide each of the detectors with a common bias voltage. A temperature stabilizer very accurately controls the ambient temperature of each of the detectors. As a result, different responses of the two detectors to the light are independent of detector ambient temperature and of the bias voltage input to the detector, but are dependent on the light received by the respective detector.
In another embodiment, the gas correlation radiometer is configured to respond to light transmitted through the region, wherein the wavelength of the light to which the radiometer responds is within a band selected as corresponding to strong infrared (IR) absorption by the target gas and also weak IR absorption by gases present in the free atmosphere as compared to the strong absorption by the target gas.
In another embodiment for use in the natural gas industry, the target gas is each of methane and ethane, and the source of light is configured to direct light into a region in which the methane and the ethane may be present. Further, there are two gas correlation radiometers. An optimized one of the radiometers is configured to respond to light transmitted through the region and having a wavelength within a band of about 1000 to about 3500 cmxe2x88x921. The other optimized one of the radiometers is configured to respond to light transmitted through the region and having a wavelength within a band of about 2970 to about 3005 cmxe2x88x921. The two optimized gas correlation radiometers are configured to simultaneously detect any respective ethane and methane in the region into which the light is transmitted so that the combined simultaneous output of the two optimized gas correlation radiometers may indicate the presence in the region of natural gas.
In a natural gas detection embodiment that is configured to be mounted on a vehicle, such as a ground-based vehicle or an aircraft, the apparatus is configured with two gas correlation radiometers, each having a separate background channel and a separate gas channel. The background channel of the methane gas correlation radiometer is configured with a blank cell to output blank methane data that varies according to whether there is methane in the region. The gas channel of the methane gas correlation radiometer is configured with a methane cell to output methane data that is independent of whether there is methane in the region. The background channel of the ethane gas correlation radiometer is configured with a blank cell to output blank ethane data that varies according to whether there is ethane in the region. The gas channel of the ethane gas correlation radiometer is configured with an ethane cell to output ethane data that is independent of whether there is ethane in the region. The sensitivity of the apparatus to methane and ethane is rendered independent of atmospheric turbulence and variability by providing a computer programmed to process the data resulting from the simultaneous detection of the ethane and methane in the region into which the light is transmitted. For the ethane data and the methane data, the computer is programmed to compute an output based on Equations (1) and (2), as follows:
output for ethane=(Ratiomeasured E/Rationull E)xe2x88x921;xe2x80x83xe2x80x83Equation (1)
wherein Ratiomeasured E is the ratio of the ethane data to the blank ethane data, and
wherein Rationull E is the ratio of the ethane data measured without the ethane cell in the respective gas channel of the respective gas correlation radiometer, to the blank ethane data measured without the blank cell in the respective blank channel of the respective gas correlation radiometer,
xe2x80x83output for methane=(Ratiomeasured M/Rationull M)xe2x88x921;xe2x80x83xe2x80x83Equation (2)
wherein Ratiomeasured M is the ratio of the methane data to the blank methane data, and
wherein Rationull M is the ratio of the methane data measured without the methane cell in the respective gas channel of the respective gas correlation radiometer, to the blank methane data measured without the blank cell in the respective blank channel of the respective gas correlation radiometer.
Embodiments of the present invention include a method of selecting an optimum center wavelength and an optimum bandpass of wavelengths of light to be processed by a gas correlation radiometer after transmission of the light through the free atmosphere in which there may be a trace amount of a target gas and in which there is likely to be at least one competitive other gas the presence of which in the free atmosphere may interfere with detection of the target gas. The optimum center wavelength and the optimum bandpass are used in optimizing a response of a gas correlation radiometer to the trace amount of the target gas. The method may include an operation of determining a set of similarity data as a function of overlap regions within a spectral region. The overlap regions are for each competitive gas and the target gas and are those regions within the spectral region in which respective absorption spectra of both the target gas and the competitive gas have absorption characteristics. The set of similarity data include a plurality of data items within each of a plurality of bandpasses, wherein one of the data items corresponds to a center wavelength within each bandpass. The method may also include an operation of determining a set of contrast data as a function of non-overlap regions within the spectral region, the non-overlap regions being for each of the competitive gas and the target gas and being those regions within the spectral region in which the first absorption spectrum has high absorption characteristics but the second absorption spectrum has low absorption characteristics. The set of contrast data include a plurality of data items within each of a plurality of bandpasses, wherein one of the data items corresponds to a center wavelength within each bandpass. Optimization is completed by selecting the center wavelength and bandpass of an infrared filter for use with the gas correlation radiometer. Such selection is based on plotting a curve for each of various bandpasses. Each curve plots the contrast data item minus the similarity data item as a function of the center wavelength. The selected center wavelength and bandpass have the highest value of contrast data item minus the similarity data item.
The invention also provides a lock-in amplifier configured to receive electronic signals produced by first and second detectors of a gas correlation radiometer. The amplifier isolates the source-modulated broadband light signal from any ambient steady-state thermal emission background that may be present along a target gas detection path.